Metastasis, the spread and growth of tumor cells from the primary site to distant organs, represents the most devastating and deadly attribute of cancer and is responsible for 90% of cancer deaths. Although a systematic understanding of metastasis biology is yet to be established, there is a growing recognition of the importance of circulating tumor cells (CTCs) as metastasis-initiating cells, which will provide a potential accessible source for early diagnosis, characterization and monitoring of cancer progression. The reliable detection and non-invasive isolation of CTCs and other nucleated cells from the blood of cancer patients, however, remains technically challenging, not only because of their extremely rare presence (as low as one in a billion or more blood cells), but also due to the level of heterogeneity in biophysical and biochemical properties.
In terms of size, CTCs and other rare nucleated cells can be as small as 5 to 8 microns, e.g., the size of red blood cells, or 8 to 18 microns, which is about the same size as human white blood cells, or over 100 microns in the form of CTC clusters. In terms of surface chemistry, the expression of epithelial cell adhesion molecule (EpCAM), a biomarker that has been widely used to target CTCs and epithelial cells in general in positive-selection methods has exhibited a large variation between clinical samples and can also be significantly down-regulated with cancer progression as a result of the epithelial-mesenchymal transition (EMT). Furthermore, tumor cells have been reported to actively interact with host cells in the microenvironment, e.g., blood cells such as white blood cells and platelets, during the development of metastasis, which may make the detection and isolation of CTCs even more challenging.